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basin is boiled up again with a volume of water equal to that
decanted, and again a third and fourth time, and filtered hot
each time until no more lead formate is dissolved. Xlae U»;id
formate will have now passed into solution and the liquid is then
evaporated down on a sand-bath of i~inK-
burner(see Fig. 69), until crystals £iplK*ar
on the surface, when the liquid is pu*- on
one side to cool. Lead formate crys*a*~
lises out in long white needles. Vit:ld
about 150 grams. In order to ototuin
pure formic acid, hydrogen sulpliW0 *s
passed over the heated lead salt. It **
carried out as follows : —
9'                The powdered salt, dried on the w*ttor-

bath, is introduced in a long layer into a sloping wide tulxs
loosely stopped at the lower end by a plug of glass wool or
asbestos.* To the lower end of the tube a receiver, in tine form
of a distilling-flask, is attached, which is protected from moisture
by a drying-tube. The salt is heated gently by moving SL fhune
along the tube whilst hydrogen sulphide, washed throug'li wnt or,
and dried by passing through a U-tube containing calcium chlor-
ide, is led over the salt in not too rapid a stream. Tlie 3<-»;nI
formate blackens, and is slowly converted into lead sulplilcle ;md
formic acid, which drops into the receiver. The acid, which retri I ns
a strong smell of hydrogen sulphide, is freed from the latter l>y
distillation over a little dry lead formate. Yield is iiearly


Glycerole monoformin.

Formic acid.
/^j.— Colourless liquid, with a penetrating smell i*c*-
sembling sulphurous acid; b. p. 100°; 1-223 at o°; solidifies
below o° to colourless crystals; m. p. 8'6° ; soluble in water ;incl
Reactions.— Tor the following tests use a neutral solutiori   j >»•<*-
pared as follows :—Boil a little lead formate with a solution   of